Methods for the synthesis of transition metal dichalcogenide (TMDC) nanoparticles

ABSTRACT

Methods of synthesizing transition metal dichalcogenide nanoparticles include forming a metal-amine complex, combining the metal-amine complex with a chalcogen source in at least one solvent to form a solution, heating the solution to a first temperature for a first period of time, and heating the solution to a second temperature that is higher than the first temperature for a second period of time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional applicationSer. No. 15/879,136, filed Jan. 24, 2018, now U.S. Pat. No. 10,883,046,which claims the benefit of U.S. Provisional Application Ser. No.62/453,780, filed Feb. 2, 2017, and U.S. Provisional Application Ser.No. 62/588,774, filed Nov. 20, 2017, the contents of which are herebyincorporated by reference in their entireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to two-dimensional (2D)materials. More particularly, it relates to 2D nanoparticles.

Description of the Related Art Including Information Disclosed Under 37CFR 1.97 and 1.98.

The isolation of graphene via the mechanical exfoliation of graphite [K.S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V.Dubnos, I.V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666] hassparked strong interest in two-dimensional (2D) layered materials. Theproperties of graphene include exceptional strength, and high electricaland thermal conductivity, while being lightweight, flexible andtransparent. This opens the possibility of a wide array of potentialapplications, including high speed transistors and sensors, barriermaterials, solar cells, batteries, and composites.

Other classes of 2D materials of widespread interest include transitionmetal dichalcogenide (TMDC) materials, hexagonal boron nitride (h-BN),as well as those based on Group 14 elements, such as silicene andgermanene. The properties of these materials can range fromsemi-metallic, for example, NiTe₂ and VSe₂, to semiconducting, forexample, WSe₂ and MoS₂, to insulating, for example, h-BN.

2D nanosheets of TMDC materials are of increasing interest forapplications ranging from catalysis to sensing, energy storage andoptoelectronic devices.

TMDC monolayers are atomically thin semiconductors of the type MX₂,where M a transition metal element (Mo, W, etc.) and X a chalcogenelement (S, Se, or Te). A single layer of M atoms is sandwiched betweentwo layers of X atoms. A MoS₂ monolayer is 6.5 Å thick. Of the 2D TMDCs,the semiconductors WSe₂ and MoS₂ are of particular interest because,while largely preserving their bulk properties, additional propertiesarise due to quantum confinement effects when the dimensions of thematerials are reduced to mono- or few layers. In the case of WSe₂ andMoS₂, these include the exhibition of an indirect-to-direct band gaptransition, with strong excitonic effects, when the thickness is reducedto a single monolayer. This leads to a strong enhancement inphotoluminescence efficiency, opening new opportunities for theapplication of such materials in optoelectronic devices. Other materialsof particular interest include WS₂ and MoSe₂.

The discovery of graphene illustrates how new physical properties mayemerge when a bulk crystal of macroscopic dimensions is thinned down toone atomic layer. Like graphite, TMDC bulk crystals are formed ofmonolayers bound to each other by van der Waals attraction. TMDCmonolayers have properties that are distinctly different from those ofthe semi-metal graphene. For example, TMDC monolayers MoS₂, WS₂, MoSe₂,WSe₂ and MoTe₂ have a direct band gap, and can be used in electronics astransistors and in optics as emitters and detectors. Group 4 to 7 TMDCspredominantly crystallise in layered structures, leading to anisotropyin their electrical, chemical, mechanical and thermal properties. Eachlayer comprises a hexagonally packed layer of metal atoms sandwichedbetween two layers of chalcogen atoms via covalent bonds. Neighboringlayers are weakly bound by van der Waals interactions, which may easilybe broken by mechanical or chemical methods to create mono- andfew-layer structures.

The TMDC monolayer crystal structure has no inversion center, whichallows access to a new degree of freedom of charge carriers, namely thek-valley index, and to open up a new field of physics: “valleytronics.”

The strong spin-orbit coupling in TMDC monolayers leads to a spin-orbitsplitting of hundreds meV in the valence band and a few meV in theconduction band, which allows control of the electron spin by tuning theexcitation laser photon energy.

The work on TMDC monolayers is an emerging research and developmentfield since the discovery of the direct bandgap and the potentialapplications in electronics and valley physics. TMDCs may be combinedwith other 2D materials like graphene and hexagonal boron nitride tomake van der Waals heterostructure devices.

A semiconductor can absorb photons with energy larger than or equal toits bandgap. This means that light with a shorter wavelength isabsorbed. Semiconductors are typically efficient emitters if the minimumof the conduction band energy is at the same position in k-space as themaximum of the valence band, i.e., the band gap is direct. The band gapof bulk TMDC material down to a thickness of two monolayers is stillindirect, so the emission efficiency is lower compared to monolayeredmaterials. The emission efficiency is about 10⁴ times greater for a TMDCmonolayer than for bulk material. The band gaps of TMDC monolayers arein the visible range (between 400 nm and 700 nm). The direct emissionshows two transitions called A and B, separated by the spin-orbitcoupling energy. The lowest energy and therefore most important inintensity is the A emission. Owing to their direct band gap, TMDCmonolayers are promising materials for optoelectronics applications.

In its multilayer form, MoS₂ is a silvery black solid that occurs as themineral molybdenite—the principal ore for molybdenum. MoS₂ is relativelyunreactive. It is unaffected by dilute acids and oxygen. MoS₂ is similarto graphite in its appearance and feel. It is widely used as a solidlubricant due to its low-friction properties and robustness. As a TMDC,MoS₂ possesses some of graphene's desirable qualities (such asmechanical strength and electrical conductivity), and can emit light,opening possible applications such as photodetectors and transistors.

For high-performance applications, flat, defect-free material isrequired, whereas for applications in batteries and supercapacitors,defects, voids and cavities are desirable.

Mono- and few-layer 2D nanosheets may be produced using “top-down” and“bottom-up” approaches. Top-down approaches involve the removal oflayers, either mechanically or chemically, from the bulk material. Suchtechniques include mechanical exfoliation, ultrasound-assisted liquidphase exfoliation (LPE), and intercalation techniques. Bottom-upapproaches, wherein 2D layers are grown from their constituent elements,include chemical vapor deposition (CVD), atomic layer deposition (ALD),and molecular beam epitaxy (MBE), as well as solution-based approachesincluding hot-injection.

A number of approaches to synthesize 2D nanosheets have been describedin the prior art, the most common of which include mechanicalexfoliation, LPE and CVD, with a small number of reports ofsolution-based approaches predominantly utilizing hot-injectiontechniques. While mechanical exfoliation provides highly crystallineflakes, the process is low yielding, offers poor thickness control andis unscalable. LPE offers a scalable route to the production of 2Dnanosheets, and may be carried out under ambient conditions using lesshazardous chemicals than other techniques. However, as with mechanicalexfoliation, it provides poor thickness control, along with low reactionyields, and produces small flakes. Poor reaction yields are also typicalof CVD syntheses. Advantages of this method include large areascalability, uniformity and thickness control. However, the quality ofthe resulting material is not comparable to that of mechanicallyexfoliated flakes, with the so-produced flakes typically being small anddisplaying poor long-term stability. Solution-based synthetic approachesare of increasing interest and have the potential to provide controlover the size, shape and uniformity of the resulting 2D materials. Yet,further improvements are required to provide the ultimate combination ofa scalable method of synthesis that generates flakes with the desiredcrystallographic phase, tunable and narrow size and shape distributions,and capped with a volatile ligand.

There are few literature reports of the colloidal synthesis of 2Dquantum dots made via a “bottom up” approach. Most are “top down”exfoliation-based methods—i.e. methods wherein a bulk material isexfoliated to provide a 2D material. Solution-based approaches for theformation of 2D flakes are highly desirable, as they may offer controlover the size, shape and uniformity of the resulting materials, as wellas enabling ligands to be applied to the surface of the materials toprovide solubility and, thus, solution processability. The applicationof organic ligands to the surface of the materials may also limit thedegradation, as observed for CVD-grown samples, by acting as a barrierto oxygen and other foreign species. The resulting materials arefree-standing, further facilitating their processability. However, thesolution-based methods thus far developed do not provide a scalablereaction to generate 2D layered materials with the desiredcrystallographic phase, tunable narrow shape and size distributions anda volatile capping ligand, which is desirable in that it can be easilyremoved during device processing. One promising reference for MoS₂ usedthe single-source precursor ammonium tetrathiomolybdate ((NH₄)₂MoS₄).[H. Lin, C. Wang, J. Wu, Z. Xu, Y. Huang and C. Zhang, New J. Chem.,2015, 39, 8492] However, the reported method produces an insolublematerial. It is contemplated that an organic-soluble material would behighly advantageous for certain applications and/or ease of use.

One of the challenges in the production of 2D layered materials is toachieve compositional uniformity, whether high-quality, defect-free, ordefect-containing material is required, on a large scale. Furtherchallenges include forming 2D flakes with a homogeneous shape and sizedistribution.

Thus, there is a need for a synthesis method that produces 2Dnanoparticles with uniform properties that can be solution-processed.

BRIEF SUMMARY OF THE INVENTION

Herein, a method to prepare nanoparticles is described. The method maybe used to produce 2D nanoparticles with uniform properties, which maybe solution-processed.

In one embodiment, the method of synthesis comprises combining a firstnanoparticle precursor and a second nanoparticle precursor in one ormore solvents to form a solution, followed by heating the solution to afirst temperature for a first time period, then subsequently heating thesolution to a second temperature for a second time period, wherein thesecond temperature is higher than the first temperature, to effect theconversion of the nanoparticle precursors into 2D nanoparticles.

In one embodiment, the first nanoparticle precursor is a metal-aminecomplex. In one embodiment, the second nanoparticle precursor is aslow-releasing chalcogen source.

In one embodiment, the method of synthesis comprises dissolving asingle-source precursor in a solvent to form a solution, heating thesolution to a first temperature for a first time period, thensubsequently heating the solution to a second temperature for a secondtime period, wherein the second temperature is higher than the firsttemperature, to effect the conversion of the single-source precursorinto 2D nanoparticles.

In one embodiment, the 2D nanoparticles are TMDC nanoparticles.

In one embodiment, the 2D nanoparticles are 2D quantum dots (QD).

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a photoluminescence contour map of MoS₂ 2D nanoparticlesprepared according to Example 4.

FIG. 2 is a Raman spectrum of MoS₂ 2D nanoparticles prepared accordingto Example 4.

DETAILED DESCRIPTION OF THE INVENTION

Herein, a method to prepare nanoparticles is described. The process canbe used to produce 2D nanoparticles with uniform properties. In oneembodiment, the 2D nanoparticles are prepared via a one-pot method.

As used herein, the term “nanoparticle” is used to describe a particlewith dimensions on the order of approximately 1 to 100 nm. The term“quantum dot” (QD) is used to describe a semiconductor nanoparticledisplaying quantum confinement effects. The dimensions of QDs aretypically, but not exclusively, between 1 to 10 nm. The terms“nanoparticle” and “quantum dot” are not intended to imply anyrestrictions on the shape of the particle. The term “2D nanoparticle” isused to describe a particle with lateral dimensions on the order ofapproximately 1 to 100 nm and a thickness between 1 to 10 atomic ormolecular layers, and wherein the lateral dimensions are greater thanthe thickness. The term “2D nanoflake” is used to describe a particlewith lateral dimensions on the order of approximately 1 to 100 nm and athickness between 1 to 5 atomic or molecular layers.

As used herein, the term “one-pot method” is used to describe a methodof synthesis wherein the nanoparticle precursors are converted to 2Dnanoparticles in a single reaction vessel.

The composition of the nanoparticles is unrestricted. Suitable materialsinclude, but are not restricted to:

graphene oxide and reduced graphene oxide;

transition metal dichalcogenides (TMDCs) such as, for example, WO₂; WS₂;WSe₂; WTe₂; MnO₂; MoO₂; MoS₂; MoSe₂; MoTe₂; NiO₂; NiTe₂; NiSe₂; VO₂;VS₂; VSe₂; TaS₂; TaSe₂; RuO₂; RhTe₂; PdTe₂; HfS₂; NbS₂; NbSe₂; NbTe₂;FeS₂; TiO₂; TiS₂; TiSe₂; and ZrS₂;

transition metal trichalcogenides such as, for example, TaO₃; MnO₃; WO₃;ZrS₃; ZrSe₃; HfS₃; and HfSe₃;

Group 13-16 (III-VI) compounds such as, for example, InS; InSe; GaS;GaSe; and GaTe;

Group 15-16 (V-VI) compounds such as, for example, Bi₂Se₃; and Bi₂Te₃;

nitrides such as, for example, h-BN;

oxides such as, for example, LaVO₃; LaMnO₃; V₂O₅; LaNbO₇; Ca₂Nb₃O₁₀;Ni(OH)₂; and Eu(OH)₂; layered copper oxides; micas; and bismuthstrontium calcium copper oxide (BSCCO);

phosphides such as, for example, Li₇MnP₄; and MnP₄.

In the aforementioned materials, adjacent layers are held together byvan der Waals interactions, which can readily be separated duringsynthesis to form 2D flakes. In alternative embodiments, thenanoparticles comprise non-layered semiconductor materials, including,but not restricted to:

Group 12-16 (II-VI) semiconductors such as, for example, ZnS; ZnSe; CdS;CdSe; CdTe;

Group 13-15 (III-V) materials such as, for example, GaN; GaP; GaAs; InN;InP; InAs; and

Group I-III-VI materials such as, for example, CuGaS₂; CuGaSe₂;CuGa(S,Se)₂; CuInS₂, CuInSe₂; Culn(S,Se)₂; Cu(In,Ga)S₂; Cu(In,Ga)Se₂;Cu(In,Ga)(S,Se)₂; CuInTe₂; AgInS₂; and AgInSe₂; and

including doped species and alloys thereof.

In some embodiments, the 2D nanoparticles are 2D nanoflakes. In someembodiments, the 2D nanoparticles are 2D QDs. QDs have widely beeninvestigated for their unique optical, electronic and chemicalproperties, which originate from “quantum confinement effects”—as thedimensions of a semiconductor nanoparticle are reduced below twice theBohr radius, the energy levels become quantized, giving rise to discreteenergy levels. The band gap increases with decreasing particle size,leading to size-tunable optical, electronic and chemical properties,such as size-dependent photoluminescence. Moreover, it has been foundthat reducing the lateral dimensions of a 2D nanoflake into the quantumconfinement regime may give rise to yet further unique properties,depending on both the lateral dimensions and the number of layers of the2D nanoflake. In some embodiments, the lateral dimensions of the 2Dnanoflakes may be in the quantum confinement regime, wherein theoptical, electronic and chemical properties of the nanoparticles may bemanipulated by changing their lateral dimensions. For example, metalchalcogenide monolayer nanoflakes of materials such as MoSe₂ and WSe₂with lateral dimensions of approximately 10 nm or less may displayproperties such as size-tunable emission when excited. This can enablethe electroluminescence maximum (EL_(max)) or photo-luminescence(PL_(max)) of the 2D nanoflakes to be tuned by manipulating the lateraldimensions of the nanoparticles. As used herein, a “2D quantum dot” or“2D QD” refers to a semiconductor nanoparticle with lateral dimensionsin the quantum confinement regime and a thickness between 1-5 atomic ormolecular monolayers. As used herein, a “single-layered quantum dot” or“single-layered QD” refers to a semiconductor nanoparticle with lateraldimensions in the quantum confinement regime and a thickness of a singlemonolayer. Compared with conventional QDs, 2D QDs have a much highersurface area-to-volume ratio, which increases as the number ofmonolayers is decreased. The highest surface area-to-volume ratio isseen for single-layered QDs. This may lead to 2D QDs having verydifferent surface chemistry from conventional QDs, which may beexploited for applications such as catalysis.

In one embodiment, the method of synthesis comprises combining a firstnanoparticle precursor and a second nanoparticle precursor in one ormore solvents to form a solution, followed by heating the solution to afirst temperature for a first time period, then subsequently heating thesolution to a second temperature for a second time period, wherein thesecond temperature is higher than the first temperature, to effect theconversion of the nanoparticle precursors into 2D nanoparticles.

In an alternative embodiment, the method of synthesis comprisesdissolving a single-source precursor in a solvent to form a solution,heating the solution to a first temperature for a first time period,then subsequently heating the solution to a second temperature for asecond time period, wherein the second temperature is higher than thefirst temperature, to effect the conversion of the single-sourceprecursor into 2D nanoparticles.

In one embodiment, the first precursor is a metal precursor. Suitablemetal precursors may include, but are not restricted to, inorganicprecursors, for example:

-   -   metal halides such as WCl_(n), (n=4-6), Mo₆Cl₁₂, MoCl₃,        [MoCl₅]₂, NiCl₂, MnCl₂, VCl₃, TaCl₅, RuCl₃, RhCl₃, PdCl₂, HfCl₄,        NbCl₅, FeCl₂, FeCl₃, TiCl₄, SrCl₂, SrCl₂·6H₂O, WO₂Cl₂, MoO₂Cl₂,        Cu Cl₂, ZnCl₂, CdCl₂, GaCl₃, InCl₃, WF₆, MoF₆, NiF₂, MnF₂, TaF₅,        NbF₅, FeF₂, FeF₃, TiF₃, TiF₄, SrF₂, NiBr₂, MnBr₂, VBr₃, TaBr₅,        RuBr₃·XH₂O, RhBr₃, PdBr₂, HfBr₄, NbBr₅, FeBr₂, FeBr₃, TiBr₄,        SrBr₂, NiI₂, MnI₂, RuI₃, RhI₃, PdI₂ or TiI₄;    -   (NH₄)₆H₂W₁₂O₄₀ or (NH₄)₆H₂Mo₁₂O₄₀;    -   organometallic precursors such as metal carbonyl salts, for        example, Mo(CO)₆, W(CO)₆, Ni(CO)₄, Mn₂(CO)₁₀, Ru₃(CO)₁₂,        Fe₃(CO)₁₂ or Fe(CO)₅ and their alkyl and aryl derivatives;    -   acetates, for example, Ni(ac)₂·4H₂O, Rh₂(ac)₂·4H₂O, Rh₂(ac)₄,        Pd₃(ac)₆, Pd(ac)₂, Fe(ac)₂, Sr(ac)₂, Cu(ac)₂, Zn(ac)₂, Cd(ac)₂        or In(ac)₃, where ac=OOCCH₃;    -   acetylacetonates, for example, Ni(acac)₂, Mn(acac)₂, V(acac)₃,        Ru(acac)₃, Rh(acac)₃, Pd(acac)₂, Hf(acac)₄, Fe(acac)₂,        Fe(acac)₃, Sr(acac)₂, Sr(acac)₂·₂H₂O, Cu(acac)₂, Ga(acac)₃ or        In(acac)₃, where acac=CH₃C(O)CHC(O)CH₃;    -   hexanoates, for example, Mo[OOCH(C₂H₅)C₄H₉]_(X),        Ni[OOCCH(C₂H₅)C₄H₉]_(X), Mn[OOCCH(C₂H₅)C₄H₉]₂,        Nb[OOCCH(C₂H₅)C₄H₉]₄, Fe[OOCCH(C₂H₅)C₄H₉]₃ or        Sr[OOCCH(C₂H₅)C₄H₉]₂;    -   stearates, for example, Ni(st)₂, Fe(st)₂ or Zn(st)₂, where        st=O₂C₁₈H₃₅;    -   amine precursors, for example, complexes of the form        [M(CO)n(amine)_(6−n)] where M is a metal and 1≤n<6;    -   metal alkyl precursors, for example, W(CH₃)₆; or

bis(ethylbenzene)molybdenum [(C₂H₅)_(y)C₆H_(6-y)]₂Mo (y=1-4).

In one embodiment, the second precursor is a non-metal precursor.Non-limiting examples include a chalcogen precursor, such as, but notrestricted to, an alcohol, an alkyl thiol or an alkyl selenol; acarboxylic acid; H₂S or H₂Se; an organo-chalcogen compound, for examplethiourea or selenourea; inorganic precursors, for example Na₂S, Na₂Se orNa₂Te; phosphine chalcogenides, for example trioctylphosphine sulfide,trioctylphosphine selenide or trioctylphosphine telluride; octadecenesulfide, octadecene selenide or octadecene telluride; diphenyldichalcogenides, for example diphenyl disulfide, diphenyl diselenide ordiphenyl ditelluride; or elemental sulfur, selenium or tellurium.Particularly suitable chalcogen precursors include linear alkyl selenolsand thiols such as octane thiol, octane selenol, dodecane thiol ordodecane selenol, or branched alkyl selenols and thiols such astert-dibutyl selenol or tert-nonyl mercaptan, which may act as both achalcogen source and capping agent. It has been found that the use of aslow-releasing chalcogen source provides controllable growth in such asynthesis method for 2D nanoparticles. In this context, a“slow-releasing chalcogen source” is defined as being a compound havinga chalcogen-carbon bond that is broken when the compound acts as achalcogen precursor in a nanoparticle synthesis reaction. In a furtherembodiment, the slow-releasing chalcogen source may initially decomposevia the cleavage of a chalcogen-chalcogen bond, then in a subsequentstep a carbon-chalcogen bond is broken when the compound acts as achalcogen precursor in a nanoparticle synthesis reaction. Suitableslow-releasing chalcogen precursors include, but are not restricted to:compounds of the form R—X—R′, wherein R is an alkyl or aryl group, X isa chalcogen and R′ is H, alkyl, aryl or X—R″ (wherein R″ is alkyl oraryl). In a particular embodiment, the slow-releasing chalcogen sourceis a slow-releasing sulfur source such as 1-dodecanethiol (DDT).

Other suitable non-metal precursors include Group 15 precursors, suchas, but not restricted to, NR₃, PR₃, AsR₃, SbR₃ (R=Me, Et, ^(t)Bu,^(i)Bu, ^(i)Pr, Ph, etc.); NHR₂, PHR₂, AsHR₂, SbHR₂ (R=Me, Et, Bu, Bu,Pr, Ph, etc.); NH₂R, PH₂R, AsH₂R, SbH₂R₃ (R=Me, Et, ^(t)Bu, ^(i)Bu,Pr^(i), Ph, etc.); PH₃, AsH₃; M(NMe)₃ where M=P, As, Sb; dimethyldrazine(Me₂NNH₂); ethylazide (Et-NNN); hydrazine (H₂NNH₂); Me₃SiN₃;tris(trimethylsilyl) phosphine; and tris(trimethylsilyl) arsine.

In one embodiment, a single-source precursor may act as both a metal andnon-metal precursor. Suitable examples of single-source precursorsinclude, but are not restricted to, alkyl dithiocarbamates; alkyldiselenocarbamates; complexes with thiuram, for example, WS₃L₂, MoS₃L₂or MoL₄, where L=E₂CNR₂, E=S and/or Se, and R=methyl, ethyl, butyland/or hexyl; (NH₄)₂MoS₄; (NH₄)₂WS₄; or Mo(S^(t)Bu)₄.

The first and second precursors are combined, or the single-sourceprecursor dissolved, in one or more solvents. The boiling point of thesolvent(s) must be high enough to enable the solvent(s) to be heated toa sufficiently high temperature to effect the conversion of the firstand second nanoparticle precursors, or the single-source precursor, tonanoparticles. In some embodiments, the one or more solvents maycomprises a coordinating solvent. Examples of suitable coordinatingsolvents include, but are not restricted to: saturated alkyl amines suchas, for example, C₆-C₅₀ alkyl amines; unsaturated fatty amines such as,for example, oleylamine; fatty acids such as, for example, myristicacid, palmitic acid, and oleic acid; phosphines such as, for example,trioctylphosphine (TOP); phosphine oxides such as, for example,trioctylphosphine oxide (TOPO); alcohols such as, for examplehexadecanol, benzylalcohol, ethylene glycol, propylene glycol; and mayinclude primary, secondary, tertiary and branched solvents. In someembodiments, the one or more solvents may comprises a non-coordinatingsolvent, such as, but not restricted to, a C₁₁-C₅₀ alkane. In someembodiments, the boiling point of the solvent is between 150° C. to 600°C., for example, 160° C. to 400° C., or more particularly 180° C. to360° C. In one particular embodiment, the solvent is hexadecylamine. Inanother embodiment, the solvent is myristic acid. If a non-coordinatingsolvent is used, the reaction may proceed in the presence of a furthercoordinating agent to act as a ligand or capping agent. Capping agentsare typically Lewis bases, for example phosphines, phosphine oxides,and/or amines, but other agents are available such as oleic acid ororganic polymers, which form protective sheaths around thenanoparticles. Other suitable capping agents include alkyl thiols orselenols, include linear alkyl selenols and thiols such as octane thiol,octane selenol, dodecane thiol or dodecane selenol, or branched alkylselenols and thiols such as tert-dibutyl selenol or tert-nonylmercaptan, which may act as both a chalcogen source and capping agent.Further suitable ligands include bidentate ligands that may coordinatethe surface of the nanoparticles with groups of different functionality,for example, S⁻ and O⁻ end groups.

In one embodiment, the solution is heated to a first temperature for afirst time period. The first temperature may be in the range 50 to 550°C., for example 150 to 450° C., or more particularly 200 to 350° C. Thefirst time period may be in the range 10 seconds to 5 hours, for example2 minutes to 2 hours, or more particularly 5 minutes to 50 minutes. In aparticular example, the solution is heated to a first temperature ofapproximately 260° C. for approximately 20 minutes.

In one embodiment, the solution is subsequently heated to a secondtemperature for a second time period, wherein the second temperature ishigher than the first temperature. The second temperature may be in therange 80 to 600° C., for example 200 to 500° C., or more particularly300 to 400° C. In a particular embodiment, the second temperature is theboiling point of the solution and the solution is heated to reflux. Thesecond time period may be in the range 5 minutes to 1 week, for example10 minutes to 1 day, or more particularly 20 minutes to 5 hours. In aparticular example, the solution is heated to a second temperature ofapproximately 330° C. for approximately 20 minutes. Increasing theduration of the heating of the solution at the second temperature mayincrease the yield and/or alter the dimensions of the resulting 2Dnanoparticles.

The 2D nanoparticles may be isolated from the reaction solution by anysuitable technique. Examples include, but are not restricted to,centrifugation, filtration, dialysis, and column chromatography.Size-selective isolation procedures may be employed to extract 2Dnanoparticles having similar dimensions and thus similar emissiveproperties.

Syntheses of nanoparticles in colloidal solutions are particularlyfavorable since they allow control over the shape, size and compositionof the nanoparticles, and may offer scalability. Colloidal nanoparticlesmay also be surface-functionalized with ligands (capping agents), wherethe ligands may be chosen to impart solubility in a range of solvents.Ligands may also be used to control the shape of the resultingnanoparticles. The inherent ligands deposited on the nanoparticlesurface during nanoparticle synthesis may be exchanged with alternativeligands to impart a particular function, such as improved solutionprocessability in a particular solvent.

The choice of reagents and the reaction parameters, such astemperature(s) and time(s), may be adjusted to control both the lateraldimensions and the thickness of the 2D nanoparticles and thus theiremissive properties, such as the wavelength (color) of light emitted.

The 2D nanoparticles produced by the methods described herein may bedissolved or dispersed in a suitable solvent to provide solutionprocessability. Solution-processable 2D nanoparticles are particularlyattractive for applications such as photoluminescent displays andlighting, electroluminescent displays and lighting, 2D heterostructuredevices, catalysis (for example, the hydrogen evolution reaction, theoxygen evolution reaction, catalytic desulfurization, etc.), sensors,and biological imaging.

One particular exemplary embodiment of the invention is a simple methodof producing 2D nanoparticles of MoS₂. First, a complex is formedcomprising molybdenum and an amine. Molybdenum hexacarbonyl may be usedas the molybdenum source. For a discussion of bonding in metal carbonylssee, e.g. C. Kraihanzel and F. Cotton, Inorg. Chem., 1963, 2, 533 and R.Dennenberg and D. Darensbourg, Inorg. Chem., 1972, 11, 72. Oleylaminemay be used as the amine source not only because it is a liquid andprovides ease of use but also because the double bond may provide somefunctional use by π-bonding to the metal center thereby aidingdissolution of the volatile Mo(CO)₆ that sublimes quite easily (see S.Ghosh, S. Khamarui, M. Saha and S. K. De, RSC Adv., 2015, 5, 38971). Theamine is preferably thoroughly degassed and then used to form asuspension of the pre-weighed molybdenum source and transferred back tothe reaction flask. Because Mo(CO)₆ sublimes easily, it cannot be placedunder vacuum and needs to be heated gently to ˜150° C. in order to formthe complex. The solution turns a greenish yellow then deep yellow/brownat 150° C. At this point, it may be heated rapidly to between about 250°C. and 300° C. DDT is then added rapidly and the solution left for acertain time.

In a further exemplary embodiment, a complex is formed comprisingmolybdenum and an amine. At 150° C., a sulfur source is added and themixture is transferred to a syringe and rapidly injected into anadditional quantity of the amine. The solution is heated to 260° C. fora first time period. The temperature is subsequently increased to refluxand held for a second time period.

Example 1: Preparation of MoS₂ Nanoparticles

0.132 g Mo(CO)₆ was added to a vial capped with a SUBA-SEAL® rubberseptum [SIGMA-ALDRICH CO., LLC, 3050 Spruce Street, St. Louis Mo. 63103]in a glovebox.

In a round-bottom flask, 14 mL oleylamine were degassed for 2 hours at100° C. and then cooled to room temperature.

10 mL of the degassed oleylamine was removed with a syringe and 2-3 mLinjected into the vial containing Mo(CO)₆ and shaken well. Using a cleansyringe/needle that was purged three times with nitrogen, theoleylamine/Mo(CO)₆ suspension was transferred back into the round-bottomflask.

A further 2-3 mL oleylamine were added to the vial containing theMo(CO)₆. It was shaken well and the contents again transferred back tothe round-bottom flask. This was repeated until all the oleylamine andMo(CO)₆ were transferred to the round-bottom flask.

The reaction mixture was warmed gently to 150° C. and the flask shakento dissolve any sublimed Mo(CO)₆.

The reaction mixture was then heated to 250° C.

0.25 mL DDT was injected rapidly.

The reaction was left for 30 minutes and a further 0.25 mL DDT wasinjected and again left for 30 minutes.

The reaction was then heated to 300° C. and 0.5 mL DDT was injected andleft for 30 minutes.

The reaction mixture was cooled to room temperature.

To isolate the product, 20 mL acetone were added and the supernatantdiscarded.

20 mL toluene were then added followed by 60 mL acetone.

The mixture was centrifuged and the supernatant discarded.

10 mL hexane were then added followed by 20 mL acetone then 10 mLacetonitrile and centrifuged. The supernatant was discarded and thesolid rinsed with acetone and finally dissolved in 5 mL of hexane. Briefsonication of the solid was needed to obtain full dissolution.

The solution was centrifuged and any remaining solids were discarded.

Example 2: Preparation of MoS₂ Nanoparticles

Synthesis was carried out under an inert N₂ environment.

0.132 g Mo(CO)₆ was added to a vial capped with a SUBA-SEAL® rubberseptum in a glovebox.

14 g octadecane were degassed for 2 hours at 100° C. in a round-bottomflask, then cooled to room temperature.

2 g hexadecylamine and 2 g octadecane were degassed for 2 hours at 100°C. in a vial, then cooled to 40-50° C. and injected into the vialcontaining the Mo(CO)₆ and shaken well.

The reaction mixture was warmed gently to 150° C. and the vial shaken todissolve any sublimed Mo(CO)₆, then cooled to room temperature to form aMo(CO)₆-amine complex.

The round-bottom flask (containing 14 g octadecane) was then heated to300° C.

The Mo(CO)₆-amine complex was warmed gently to ˜40° C. until the solidsmelted, and 1.5 mL 1-dodecane thiol (DDT) were added. It was thenimmediately loaded into a syringe and rapidly injected into theround-bottom flask. The temperature was adjusted to ˜260° C.

The reaction mixture was left for 8 minutes at 260° C.

To isolate the product, 40 mL propanol mixed with 10 mL acetonitrilewere added, centrifuged at 4000 rpm for 5 minutes and the supernatantdiscarded.

Example 3: Preparation of MoS₂ Nanoparticles

In a 200-mL vial, hexadecylamine (10 g) and hexadecane (50 mL) weredegassed under vacuum at 80° C. The hexadecylamine/hexadecane solutionwas added to Mo(CO)₆ (0.66 g) in a 250-mL round-bottom flask, andstirred at 120° C. to form a solution (“solution A”).

In a 1-L round-bottom flask, hexadecane (50 mL) and hexadecylamine (5 g)were heated under vacuum at 80° C. for 1 hour. The solution was heatedto 250° C., under N₂, to form a solution (“solution B”). At 250° C.,5-mL portions of solution A (maintained at 120° C.) were added tosolution B every 5 minutes for 1 hour to form a solution (“solution C”).

1-dodecanethiol (7.5 mL) was subsequently added slowly to solution C at250° C., over 1 hour, using a syringe pump, before stirring for afurther hour at 250° C. The solution was cooled to 60° C., then acetone(400 mL) was added, followed by centrifugation. The residual solids weredispersed in hexane (125 mL).

Example 4: Preparation of MoS₂ 2D Nanoparticles

In a nitrogen-filled glove box, Mo(CO)₆ (0.132 g) was added to a vialcapped with a Suba-Seal® rubber septum.

Hexadecylamine (4 g) was degassed at 100° C. for 2 hours, in a vial,then cooled to 40-50° C. and injected into the vial containing theMo(CO)₆, then shaken well.

The reaction mixture was warmed gently to 150° C. and the vial shaken todissolve any sublimed Mo(CO)₆, forming an Mo(CO)_(6-x)-(amine)_(x)complex (where 1≤x<6), and maintained just above the melting point ofthe solution. Separately, hexadecylamine (14 g) was degassed at 100° C.for 2 hours, in a round-bottom flask, then cooled to room temperature.

The round-bottom flask containing the hexadecylamine was heated to 300°C.

1-dodecanethiol (1.5 mL) was added to the Mo(CO)_(6-x)-(amine)_(x)complex, then the mixture was immediately transferred to a syringe andrapidly injected into the round-bottom flask containing thehexadecylamine. The temperature was adjusted to ˜260° C. and held for 40minutes.

The temperature was then raised to reflux (330° C.) and held at thattemperature for 20 minutes until a black precipitate formed.

The flask was cooled to 60° C. and toluene (30 mL) was added. Themixture was centrifuged at 7000 rpm for 5 minutes and the black materialwas separated and discarded. The supernatant was dried under vacuum,then acetonitrile (50 mL) was added, warmed, and the top clear layer wasdecanted and discarded to leave an oily layer. The process was repeatedthree times to remove excess hexadecylamine. The material was finallydissolved in propanol and filtered through a 0.2-μm PTFE filter.

The solution exhibited bright blue emission. The PL contour map (seeFIG. 1) shows the emission wavelength (x-axis) plotted againstexcitation wavelength (y-axis) for the MoS₂ 2D nanoparticle solution.The material showed excitation wavelength-dependent emission, with thehighest intensity emission centered around 430 nm when excited at around370 nm.

The Raman spectrum (FIG. 2) shows peaks at 375 cm⁻¹ and 403 cm⁻¹, whichare indicative of MoS₂. Note: the peaks at around 300 cm⁻¹ and 500 cm⁻¹are from the background spectrum.

The foregoing presents particular embodiments of a system embodying theprinciples of the invention. Those skilled in the art will be able todevise alternatives and variations which, even if not explicitlydisclosed herein, embody those principles and are thus within the scopeof the invention. Although particular embodiments of the presentinvention have been shown and described, they are not intended to limitwhat this patent covers. One skilled in the art will understand thatvarious changes and modifications may be made without departing from thescope of the present invention as literally and equivalently covered bythe following claims.

What is claimed is:
 1. A method of preparing a transition metaldichalcogenide (TMDC) nanoparticle, the method comprising: forming ametal-amine complex; combining the metal-amine complex with a chalcogensource in at least one solvent to form a solution; heating the solutionto a first temperature for a first period of time; and heating thesolution to a second temperature that is higher than the firsttemperature for a second period of time, wherein the chalcogen source isan organo-chalcogen compound that supplies a chalcogen for the formationof a TMDC nanoparticle via the cleavage of a chalcogen-carbon bond. 2.The method of claim 1, wherein the metal-amine complex has the formulaM(CO)_(n)(amine)_(6−n), where M is a metal; 1≤n<6; and the amine is anunsaturated fatty amine.
 3. The method of claim 2, wherein theunsaturated fatty amine is oleylamine.
 4. The method of claim 2, whereinthe unsaturated fatty amine is hexadecylamine.
 5. The method of claim 1,wherein the organo-chalcogen compound is an alkyl thiol.
 6. The methodof claim 5, wherein the alkyl thiol is 1-dodecanethiol.
 7. The method ofclaim 1, wherein the organo-chalcogen compound is an alkyl selenol. 8.The method of claim 7, wherein the alkyl selenol is octane selenol. 9.The method of claim 1, wherein the metal of the metal-amine complex ismolybdenum.
 10. The method of claim 1, wherein the metal of themetal-amine complex comprises a metal carbonyl.
 11. The method of claim10, wherein the metal carbonyl is molybdenum hexacarbonyl.
 12. Themethod of claim 1, wherein the at least one solvent is a coordinatingsolvent.
 13. The method of claim 12, wherein the coordinating solvent ishexadecylamine.
 14. The method of claim 1, wherein the at least onesolvent is a non-coordinating solvent.
 15. The method of claim 1,wherein the transition metal dichalcogenide nanoparticle has lateraldimensions between 1-100 nm.
 16. The method of claim 1, wherein thetransition metal dichalcogenide nanoparticle has lateral dimensionsbetween 1-10 nm.
 17. The method of claim 1, wherein the metal of themetal-amine complex is tungsten.
 18. The method of claim 1, wherein theorgano-chalcogen compound has the formula R—X—R′, wherein R is an alkylgroup or an aryl group, X is a chalcogen, and R′ is H, an alkyl group,or an aryl group.
 19. The method of claim 1, wherein the TMDCnanoparticle is a MoS₂ nanoparticle.
 20. The method of claim 1, whereinthe first temperature is in the range of 50 to 500° C. and the firstperiod of time in the range of 10 seconds to 5 hours; and the firsttemperature is in the range of 80 to 600° C. and the first period oftime in the range of 5 minutes to 1 week.